I don't know what's wrong with my iphone but I can't join hotspot.. Any idea??

You can try resetting your iPhone by simultaneously pressing and holding the Home and Sleep/Wake buttons until you see the Apple Logo. This can take up to 15 seconds so be patient and don't release the buttons until the logo appears.

Try again to see if the problem persists.

If still a problem, try deleting the hotspot (select the ⓘ, then tap Forget this Network) and adding it again.

Yes, I mean reboot the device but follow the instructions I gave you exactly.

Also try resetting your router.

Settings > General > Reset > Reset Network Settings